*** PHP 5 users ***
If your server is running on PHP 5.2 I recommend Ultimate Seo Urls 5 / 5 PRO instead - a different, far more modern and capable contribution (producing the same seo urls if need be) with a seperate support topic.
http://forums.oscommerce.com/topic/3...-by-fwr-media/

STABLE VERSIONS:
My 2.1d ORIGINAL updated is the current stable version, this version should take over that mantle as it is massively improved but needs validation by usage.

BEWARE OF BROKEN VERSIONS:
A particular example is 2-2.2d-5 which breaks the core cache/database funtionality

What's new:

A total rewrite of the same codebase cleaned out from top to bottom.

2400 lines of code reduced to just 1016 (this is with added functionality NOT less) for faster parsing + easier to read.

Propper validation added for ALL Seo Urls

404 page functionality added for inexistant products/categories etc.

Redundant database class removed.

Replicated osCommerce functions removed.

Improved logic on 301 redirects.

Stripped the unnecessary code from the cumbersome cache methods.

Added improved delete logic.

Includes ALL bug fixes from the stable 2.1d ORIGINAL updated version I created.

Complete tidying of the messy code, neat indentation etc. much easier to read. (code should be viewed with word wrap off).

Removed the cumbersome commenting written specifically for "phpDocumentor" - while I am a fan of this type of commenting it is best used in small file modular structures not huge files containing 2400 lines making the file impossible to read/follow.

More...